Conductive compositions have been developed for various purposes including facilitating electrostatic discharge and as a current transfer medium on printed circuit boards. Recently, conductive compositions have been used, instead of conventional conductive wires, as part of electrical systems. In particular, conductive compositions have been used as a means for conducting current from a power source to current operated modules on a variety of diverse objects such as printed circuit boards, wearing apparel and children's toys.
Examples of desirable electrical systems which use conductive compositions are disclosed in U.S. Pat. No. 5,455,749 which is incorporated by reference herein and which is commonly owned with the present application. The '749 patent discloses electrical systems which include a power supply, one or more current operated modules and conductive composition for connecting the power supply to the current operated module so that current generated by the power supply can be delivered to the current operating module. In one embodiment, the conductive composition may be colored where the coloring constitutes a substantially nonconductive portion of the conductive composition. In another embodiment, the conductive composition is arranged on an object and forms at least a portion of a design thereon. In another embodiment, the conductive composition includes features which render it sufficiently durable to withstand multiple washes without cracking, substantial resistance build-up or other failure. The aforementioned properties of the conductive compositions disclosed in the '749 patent are desirable in various applications.
Additional desirable electrical systems which use conductive compositions are disclosed in U.S. Pat. No. 5,626,948 which is also assigned to the owner of the present invention. The disclosure in the '948 patent is also incorporated by reference herein. The '948 patent particularly discloses a multilayer conductive composition which can be used as part of an electrical system. Since the multilayer conductive composition disclosed in the '948 patent is an improvement over prior art conductive compositions in that it can be used for "vertically conductive" applications (i.e., applications where it may be desirable to have a bottom layer of the conductive composition with a higher conductivity than a top layer thereof so that the top layer cannot conduct a substantial amount of current along the surface thereof). The invention disclosed in the '948 patent is also useful when it is necessary for the conductive composition to be a desired color.
It is also known in the art to provide substantially clear conductive compositions which have been useful for electrostatic discharge applications. For example, the substantially clear conductive composition may be spray coated around a computer workstation to avoid electrostatic build-up that may interfere with proper operation of an associated computer.
U.S. Pat. No. 5,203,975 to Richardson discloses a process for cathodic electrode deposition of a clear coating over a layer of conductive paint. However, the '975 patent does not teach that the clear coating should be electrically conductive. Thus, it clearly does not teach the use of a conductive composition as part of an electrical system.
Clear conductive compositions have also been used in the prior art as resistive circuit elements that form part of a windshield defrosting system. In this regard, clear conductive compositions have been applied directly to the surface of an automobile windshield. In such a system, when it is desired to defrost the windshield, a switch is closed inside the automobile which permits current to flow from a power source of the automobile through the clear conductive composition and then to ground. In such a circuit, the clear conductive composition function as a resistor which dissipates a variable amount of heat in direct proportion to the amount of current forced to flow through the resistive element (i.e., through the clear conductive composition traces on the windshield). The clear conductive composition traces in the automobile windshield defroster systems do not provide an electrical current path between the power source and a current responsive circuit element. Further, there is no trigger point which includes an open circuit area in the clear conductive composition whereby current is permitted to flow through the clear conductive composition to a responsive circuit element upon placement of an additional conductive object across the open circuit area.
Accordingly, the prior art has failed to provide an electrical system, or a trigger circuit for use as part of an electrical system, where a substantially clear conductive composition is arranged on a substrate for providing an electrical current path between a power source and at least one responsive circuit element. Moreover, the prior art has failed to provide such a system where the clear conductive composition is used as part of a switch (i.e., a trigger point).